Talk
about a tough microbe. Deinococcus radiodurans earned a place in
the Guinness Book of World Records as "the world's toughest
bacterium" for its ability to survive enormous doses of radiationa
thousand times more than a person could. Although radiation will damage
its DNA, the microbe can put the shattered pieces back together.

This remarkable ability to mend DNA is a puzzle. Scientists have not
been able to explain its origins or identify genes in D. radiodurans
that confer superiority in repairing DNA.

Now, a study in today's issue of Science suggests that D. radiodurans
survives because its genome is arranged in densely packed rings, called
toroids. These donut-like shapes keep shards of DNA in close proximity
after a dose of shattering radiation, allowing the microbe to make repairs
without having to hunt around for hundreds of loose fragments.

"When they are packed in this very dense toroidal form, even when
they are broken into 150 to 200 pieces, these pieces stay densely packed,"
says Abraham Minsky, a chemist at Israel's Weizmann Institute of Science
in Rehovot and leader of the research. "The fragments do not diffuse,
and the right sequence is maintained."

Minsky and his colleagues in the United States scanned the bacteria with
an electron microscope to observe their chromosomes. Donut-shaped DNA
packaging is rare, known to occur mainly in D. radiodurans and
a few inert bacterial spores and spermwhose job is to protect and
serve up packets of genetic material.

Yet if the arrangement is so potent, why isn't it more common among microbes?
Perhaps because gobs of energy are expended on maintaining the rings.

The study has drawn mixed reactions.

David Schwartz, a geneticist at the University of Wisconsin-Madison,
who has studied D. radiodurans, called the genome configuration
"the only possible explanation" for its immunity from radiation.
"It's totally simple, totally cool," says Schwartz.

But John R. Battista, a Louisiana State University microbiologist who
also works with D. radiodurans, wasn't convinced that genome shape
explains the resistance. "I personally don't feel this paper has
made the connection between this shape and radio-resistance," says
Battista. "It's interesting and merits consideration, but I don't
think they've put the nail in the coffin," he adds.

The group did not look at other species of deinococcus to see if they
also had toroid chromosomes, says Battista. Nor did they examine other
radioactive-resistant microbes, including a distant relative of D.
radiodurans called rubrobacter. If these bacteria lack the donut-shaped
DNA, something else must explain their DNA repair ability.

Indeed, Battista and his colleagues believe D. radiodurans's toughness
stems from proteins it generates, not the configuration of its DNA. But
so far, he added, such a molecule has not yet been reported in the literature.

Jonathan Eisen, an evolutionary biologist at The Institute for Genomic
Research in Rockville, Maryland, also has questions about the paper's
conclusions. He would like to see a side-by-side comparison with other
radiation-resistant bacteria, as well as comparisons to species harmed
by radiation.

"We need to know whether or not radiation-sensitive bacteria
have the same type of structures," Eisen says.

Minsky's team believes that while chemistry may help some organisms resist
radiation damage, D. radiodurans's defenses are a product of its
architecture. "Being exposed to [radiation] resulting in chromosomes
being shattered to 200 fragments per chromosome is a situation that cannot
be handled solely by enzymes," he says.

Some researchers hope to use D. radiodurans to clean up toxic
waste sites. One potential strategy is to modify the bacterium to carry
genes from other microbes that neutralize toxins, like mercury and the
organic chemical toluene.